The present invention relates to radar range-Doppler imaging. Time resolution of radar signals (which corresponds to range resolution) requires large signal bandwidth while fine Doppler resolution (which corresponds to range or “closing” rate or speed) requires long signal duration. This combination can be achieved in radar systems by continuously changing or “sweeping” the frequency of the transmitted electromagnetic signal. Such frequency sweeps may be linear with time (change frequency at a constant rate) or they may be nonlinear. One possible way to accomplish frequency sweeping is to transmit the electromagnetic radar signal in the form of signal pulse train in which the pulses vary in frequency from pulse to pulse over an interval that is the reciprocal of the desired time resolution and over a time interval that is the reciprocal of the desired frequency resolution. Time resolution of signals, such as, for example, radar signals, requires large signal bandwidth and fine Doppler resolution of such signals requires long signal duration. Attempts to achieve a combination of fine time resolution and fine Doppler resolution typically use pulse trains in which pulses vary in frequency from pulse to pulse over a frequency interval that is the reciprocal of the desired time (i.e. range) resolution and over a time interval that are the reciprocal of the desired frequency Doppler resolution. With rotating objects, however, there are spreads in Doppler values that must be evaluated and that impose certain limits on pulse train duration.
A radar target object may be stationary, but contain portions which rotate. This will be true, for example, for a stationary helicopter having rotating blades. Such rotating blades will include portions which are advancing toward the radar while other portions recede from the radar. For rotating objects, Doppler spreads of signals must be continually evaluated to provide clear radar images and, as a result, limitations are required on pulse train duration in order to return satisfactory images. It should be noted that an actual physical image may not be displayed, but the resulting quantity may be described in terms of its range and range rate (i.e. Doppler) properties.
Radar signals are always accompanied by undesired noise. In order to reduce or ameliorate the effects of such noise, the processing of radar signals reflected from a target uses a filter matched to the amplitude-time distribution of the transmitted pulses, which enhances the desired reflections from the target relative to the unwanted noise. Such matched filtering is described, for example, in U.S. Pat. No. 5,151,702 issued Sep. 29, 1992 in the name of Urkowitz. The result of the amplitude-time matched filtering is a range-Doppler map or “image”. Due to the matched filtering operation, a point object having constant radial velocity (termed “closing” speed or velocity regardless of whether the target approaches or recedes) will be represented by signals exhibiting a spread in range and Doppler value. This phenomenon is called “blurring” and results from the use of the matched filter. Thus, a body having several different motions will have a “blurred” radar echo in which the targets and motions are not well resolved. The blurring introduces an uncertainty into the determination of the location and speed of the target.
Improved or alternative radar image deblurring is desired.